In the field of graphic arts and medical diagnosis, there have been concerns in processing of photographic film with respect to effluent produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environmental protection and saving of floor space. A photothermographic dry imaging material for photographic use, capable of forming images by adding only heat, has been made practicable, and rapidly put into wide use.
A photothermographic material itself (hereinafter, referred to as thermodevelopable material or photosensitive material) has been proposed for a long time. For example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, and by D. Morgan, “Dry Silver Photographic Material” in IMAGING PROCESSES and MATERIALS, Neblette's Eighth Edition, edited by J. M. Sturge, V. Walworth, and A. Shepp (1989) page 279, a photothermographic materials comprising a support provided thereon a organic silver salt, light-sensitive silver halide grains and a reducing agent are described. The photothermographic material provides a simply and environment-friendly system for users, without using any processing solution.
These photothermographic materials comprise a light-sensitive layer containing light-sensitive silver halide grains as a photosensor and an organic silver salt as a silver ion source, which are thermally developed with a reducing agent at a temperature of 80 to 140° C. to form images, with no need to be subjected to fixing.
In photothermographic materials containing an organic silver salt, however, silver halide grains together with a reducing agent easily results in fogging during storage time prior to thermal development. Furthermore, there are problems in that the photothermographic materials, after exposure, are usually developed without being fixed and the silver halide, organic silver salt and reducing agent concurrently remain in the layer so that metallic silver is thermally or photolytically produced, and after storage over a long period of time, deteriorating image quality, such as silver image tone, results.
Techniques have been disclosed for solving such problems in JP-A Nos. 6-208192 and 8-267934 (hereinafter, the term JP-A refers to an unexamined, published Japanese Patent Application); U.S. Pat. No. 5,714,311 and references cited therein. These disclosed techniques have desired effects to some extent but are not sufficient as a technique satisfying the level required in the market.
The photothermographic material is usually processed by a thermal development apparatus forming images under applied stable heat to the photothermographic material by a so-called a thermal developing processor. As described above, a large number of these thermal development apparatuses have been supplied to the market with the recent trend toward photothermographic material. However slip property between the photothermographic material and conveyance rollers or parts of a developing machine for the material may change by the condition of temperature and humidity, resulting in problems of inferior transportability and unevenness in developing. There is also a problem of density variation over time in the photothermographic material. It has been proved that these problems are observed markedly on the photothermographic material which is image exposed by a laser light and developed by heat to form images. Further, in recent years it has been demanded to miniturize laser imagers and to speed up the processing.
Thus, it is desired to improve the performance of the photothermographic material. A heated drum method has the advantage easily miniturizing a thermal development apparatus compared to a horizontal conveyance method, but it tends to produce problems of powder dust, unevenness in developing and roller marks. The use of minute average grain size silver halide enhances covering power as described in JP-A 11-295844 and 11-352627, and the use of a contrast increasing agent, such as a hydrazine compound and a vinyl compound, are also effective to obtain sufficient density on the photothermographic material for high-speed processing. However, problems of a wider density variation (printout property) in thermal development and a more pronounced unevenness after developing are observed when said technique is applied. Although printout performance is possible to be improved by decreasing the amount of a reducing agent or decreasing of silver coverage, the problem of reduction of image density over time has been noted. Further, a problem in which the silver image color tone differs from that of the current wet type X-ray film has also occurred due to the use of miniturized silver halide grains.
In addition thereto, further enhanced image quality has been desired as a perpetual theme for photothermographic materials. Specifically in the field of medical diagnostic imaging, further enhanced image quality is desired to enable more precise diagnosis.